The URL of the skins directory.
This is where MediaWiki tells the browser to get the styles from.
This settings became less important or - depending on setup and the skin used - it even became useless for MediaWiki 1.17+, where styles are generally loaded through ResourceLoader modules.

For wiki families with multiple languages, it may be a good idea to set this URL, in all your wiki languages, to just one directory or server. That way the browser will only have to load the CSS once for every wiki.

If you change where on your filesystem the skins directory is, you may also have to change $wgStyleDirectoryManual:$wgStyleDirectory. $wgStylePath only controls where the browser gets the styles from, not where MediaWiki looks for the style files.